UPSEE Solved Model Paper 6

Given- Volume of helium at STP= 50mL
We are asked to find out the number of molecules in 50 mL helium at STP.
We know that 1 mole of any gas occupies 22.4 liters of volume. Thus the number of moles in 50ml can be calculated as,
1 mole - 22.4 L (or 22400 mL)
? - 50 mL
By cross multiplying we get

50×1

22400

Also, number of molecules = Number of moles × Avogadro number ----(i)
We know that number of moles = 0.0022 and Avogadro number = 6.022 × 1023
Substituting the values in equation (i) we get,
Number of molecules = 0.0022 × 6.022 × 1023 = 1.34 × 1021
Therefore the number of molecules in 50mL helium at STP is 1.34 × 1021